Yes — unused credits roll over from one billing period to the next.
How rollover works
Your plan's credit allotment is added to your balance at the start of each billing cycle
Unused credits from previous cycles stay with you — they don't reset
Your total balance = current cycle's credits + any leftovers from past cycles
If you cancel
Your existing credits stay in your account if you cancel your subscription — they're preserved but can't be spent until you have an active subscription again. Resubscribing unlocks them immediately.
For more, see Do credits expire if I don't use them?